The present invention relates to a color correction device for correcting distortion in color caused by a color signal distortion characteristic generated by an internal circuit of a receiver and for correcting a gamma characteristic. More particularly, the present invention relates to a color correction device for adaptively correcting the color distortion according to the average luminance level of an input image signal. This application for a color correcting device is based on Korean Patent Application No. 96-36136, which is incorporated by reference herein for all purposes.
In general, a cathode ray tube (CRT) of a color television receiver displays an image whose color is different from the original color of the image transmitted by a broadcasting station. This is due to the color signal distortion characteristic generated by the internal circuits of the receiver and the color reproduction characteristic of the CRT (commonly referred to as the "gamma" characteristic).
In order to solve the above drawback, a conventional color correction device of FIG. 1 corrects the color distortion of an image signal, which is caused by the color signal distortion characteristic generated by an internal circuit of a receiver and by the gamma characteristic. The color correction device of FIG. 1 includes a coefficient correction table 10 for storing a set of coefficient correction values to be used to correct the color signal distortion characteristic of the receiver, as well as a gamma correction table 12 for storing a set of gamma correction values to be used to correct the gamma characteristic. A first operator 14 corrects the level of an input color signal based on the coefficient correction values stored in the coefficient correction table 10, and a second operator 16 corrects the level of the coefficient-corrected color signal output from the first operator 14 based on the gamma correction values stored in the gamma correction table 12. Memory 20 stores a plurality of coefficient correction values (coefficient correction value #1, coefficient correction value #2, . . . ) and a plurality of gamma correction values (gamma correction value #1, gamma correction value #2, . . . ). A color correction controller 18 is provided for selectively outputting one set of coefficient correction values and one set of gamma correction values, each stored in the memory 18, to the coefficient correction table 10 and the gamma correction table 12, respectively, according to the color signal distortion characteristic and the gamma characteristic.
FIGS. 2A and 3A show examples of the color correction using the coefficient correction values stored in the memory 20. Here, the X-axis represents the level of an input color signal, and the Y-axis represents the level of a coefficient-corrected color signal. In FIG. 2A, the level of the color signal is linearly corrected. In FIG. 3A, the level of the color signal is non-linearly corrected. The coefficient correction values stored in the memory 20 are determined depending on the state of the input color signal. Here, the state of the input color signal refers to the distribution state of the input color signal in a chromaticity diagram. The color correction controller 18 compares the distribution state of the input color signal with a desirable color distribution state to select, from the memory 20, a set of coefficient correction values for correcting the deviation obtained through the comparison. Ordinarily, the coefficient correction table 10 provides distinct coefficients for each color signal to separately correct red (R), green (G), and blue (B) color signals.
FIGS. 2B and 3B show examples of color correction using the gamma correction values stored in the memory 20. Here, the X-axis represents the level of an input color signal, and the Y-axis represents the level of a gamma-corrected color signal. As shown in FIG. 2B, the gamma level of the color signal is linearly corrected. FIG. 3B shows gamma level of the color signal as being non-linearly corrected. The gamma correction values stored in the memory 20 are determined according to the gamma characteristics of the CRT. Such gamma characteristics are known a priori based on information provided by a CRT manufacturing company. Ordinarily, the gamma correction table 12 provides distinct gamma correction values for each color signal to separately correct red (R), green (G) and blue (B) color signals.
The color correction controller 18 selectively reads out of the memory 20 a set of coefficient correction values and a set of gamma correction values according to the state of the input color signal and the gamma characteristic of the CRT, and stores the selected values in the coefficient correction table 10 and the gamma correction table 12, respectively.
The conventional color correction device of FIG. 1 can correct the color distortion caused by the color signal distortion characteristic of the internal circuit of the receiver and the gamma characteristic of the CRT. However, the conventional color correction device cannot respond adaptively to the change in the luminance level of an image signal.
For example, when a scene is dark on the whole, increasing the average level of a luminance signal makes contrast high, so that objects in the scene are more noticeable. Meanwhile, a very bright scene can cause eye strain. However, this eye strain can be reduced by lowering the average level of the luminance signal.
Thus, in a color correction device, it is desirable to perform the color correction adaptively according to the average level of the luminance signal.